1. Field of the Invention
The present invention relates to a bicycle transmission mechanism, more particularly to a bicycle hub transmission mechanism with a seal member.
2. Description of the Related Art
Referring to FIG. 1, a conventional free-wheel hub transmission mechanism is shown to include an axle 2 and a spoke-mounting barrel 1 mounted rotatably around the axle 2 in such a manner that a right axle portion 2R of the axle 2 projects outwardly from a right end portion 100 of the spoke-mounting barrel 1. A sprocket mounting member 3 is sleeved around the right axle portion 2R of the axle 2 adjacent to the right end portion 100 of the spoke-mounting barrel 1. The sprocket-mounting member 3 includes an outer shell 31, a bearing unit 4, a ratchet mechanism 7, a bearing retention member 5, a seal member 6, and a locking ring 302.
The outer shell 31 has a proximate end portion which is coupled with the right end portion 100 of the spoke-mounting barrel 1 and upon which a sprocket wheel 301 is mounted (only one is shown), and a distal threaded portion 303. The bearing unit 4 is disposed within the outer shell 31 around the right axle portion 2R of the axle 2 inboard to the distal threaded portion 303 of the outer shell 31. The ratchet mechanism 7 is disposed within the outer shell 31 inboard to the bearing unit 4, and is operably coupled with the outer shell 31 so as to permit synchronous rotation of the outer shell 31 with the spoke-mounting barrel 1 only in a forward direction. The bearing retention member 5 is sleeved fittingly around the right axle portion 2R of the axle 2 outboard to the bearing unit 4, and has a cross section smaller than that of the outer shell 31, thereby defining annular space (X) therebetween. The bearing retention member 5 further has a constricted left portion 505 that abuts against the bearing unit 4, and an enlarged right portion 506 which defines an inner shoulder 500 in cooperation with the constricted left portion 505. The locking ring 302 is sleeved around the bearing retention member 5, and is fastened threadedly to the distal threaded portion 303 of the outer shell 31. A rear fork member (not shown) abuts against a right end surface of the bearing retention member 5 to prevent removal of the bearing retention member 5 from the axle 2. The seal member 6 is disposed within the outer shell 31 between the bearing unit 4 and the inner shoulder 500 of the bearing retention member 5, and includes an L-shaped cross-sectioned metal ring 601 which is in tight engagement with the constricted left portion 505 of the bearing retention member 5, and a rubber ring 602 provided on the metal ring 601 in such a manner that the outer periphery of the rubber ring 602 contacts an inner wall surface of the distal threaded portion 303 of the outer shell 31 such that the seal member 6 isolates hermetically the annular space (X) with respect to the bearing unit 4.
When the outer shell 31 rotates with respect to the axle 2, a small clearance may be present between the outer periphery of the rubber ring 602 and the inner wall surface of the outer shell 31 in the event that the outer periphery of the rubber ring 602 does not contact any crest of the thread, and rather extends into a root of the thread. Under this condition, dust, oil and water may enter the interior of the outer shell 31 via the clearance.
In order to avoid the aforesaid disadvantage, a larger rubber ring has been proposed such that the outer periphery of the rubber ring 602 will tightly contact the inner wall surface of the outer shell 31 of the sprocket-mounting member 3. However, a considerable friction force is incurred between the outer shell 31 and the rubber ring 602 during rotation of the outer shell 31, thereby hindering smooth rotation of the sprocket-mounting member 3 relative to the axle 2.